1. Field of the Invention
The invention is generally related to the building of large vessels and more particularly to the launching of such vessels.
2. General Background
Construction of large marine vessels typically takes place on skidways from which the vessel is launched into a waterway. The skidways are sloped in order to allow the vessel to slide, mostly through the effect of gravitational force. Construction directly on the skidways is due mostly to the significant weight of marine vessels and the necessity of getting the vessel in the water. The vessel is typically supported on cradles that rest directly on the skidways. The cradles are temporarily secured to the skidways or anchored to suitable supports on land by means of tiebacks to prevent the vessel from launching prematurely. Once the vessel is completed, it is launched by releasing the tieback restraining elements that connect the cradles to the supports. The vessel and cradles go into the water where the cradles separate from the vessel as the vessel gains buoyancy and reaches a stable floating position. The use of preassembled sections or modules is common in marine vessel construction. Vessel modules are constructed in buildings that provide protection from the weather and are transported to the launch site, where they are connected together. The usual launch procedure involves activating mechanical releases which are integral parts of the tieback system. To assist the launch, positive horizontal pressure is typically provided on the marine vessel or its cradles, usually by means of hydraulic jacks. The mechanical release/tieback system typically consists of a central spine or shaft with a release mechanism and is connected to a large counterweight. The individual cradle support release devices are also connected to the spine. Once the spine release is activated, the counterweight places the spine in motion. The spine movement activates the triggers in the cradle tiebacks nearly simultaneously. The release of the tiebacks and the horizontal pressure on the vessel or cradles by jacks or other means breaks the static friction between the cradles and the skidways, initiating the vessel's motion down the skidways and into the waterway. Conventional mechanical release systems have advantages and disadvantages. Mechanical release systems have proven reliable and adequate in the past but are expensive and time consuming to construct and require fabrication to close tolerances. The mechanical release systems must be maintained throughout the life of the launch facility. The mechanical release systems must be adjusted with every launch to suit the size of the marine vessel being constructed. There is no positive way to test-release the mechanism prior to the actual launch. It can be seen that there is a need for improvement over current mechanical release systems.